1. Field of the Invention
The present invention relates to a network system in which plural frame relay networks are connected to an ATM (Asynchronous Transfer Mode) switch, to a traffic control method in the network system and to a frame relay switch in each of the plural frame relay networks.
2. Description of the Related Art
Recently, the B-ISDN (Broadband Integrated Service Digital Network) is rapidly introduced as the LAN (Local Area Network) is made a high speed and a multimedia. In data communication, as a first step for the next generation, cases increase in that an ATM network is used as a repeater for frame relay networks.
FIG. 16 is a whole structural view of a network system which an ATM network is used as a repeating network for plural frame relay networks. In FIG. 16, frame relay networks 1, 2 are respectively connected with an ATM network 3 through each IWF (InterWork Function) 4 which disassembles a frame into cells. The frame relay networks 1, 2 are also respectively connected with terminals 5, 6 for users in the network system. In the ATM network, a VP (Virtual Path) which plural VCs (Virtual Channel) are put together is set, and communication channels (VCC: Virtual Channel Connection) for respective VCs are set. A maximum transmission rate (PCR: Peak Cell Rate) which cells can be transmitted is also set every VCC. Then, when a number of cells transmitted from the IWF 4 to the ATM network 3 exceeds the PCR set for the ATM network 3, the ATM network 3 discards cells so as to avoid or release congestions in the ATM network 3. When a cell (frame) is discarded in the ATM network 3, no response frame to the discarded cell (frame) is transmitted to the terminals 5, 6. Thus, when the user of terminal 5 or 6 receives no response frame concerning the frame transmitted by the user for a predetermined time, the user determines that no frame reaches a transmitted terminal, and transmits the same frame as said discard cell (frame) again.
The above-described network system, however, has the following problems.
That is, a number of cells transmitted from the IWF 4 to the ATM network 3 exceeds the PCR, whereby cells (transmitting frames) are frequently discarded. Thus, the terminal 5 or 6 which has transmitted the discarded frame (cell) re-transmits the transmitting frames frequently, As the result, there was a possibility in that congestions occur in the frame relay networks FR1, FR2, too.
Further, frames are re-transmitted from the terminals 5, 6 caused by that cells (frames) are discarded in the ATM network 3, whereby processes increase in the frame relay networks FR1, FR2. Thus, there was a possibility in that a response deteriorates between the terminal 5 and the terminal 6 (end to end).